2.5.Â Memory Management

Each process has its own private address space.
The address space is initially divided into three logical segments:
text,
data,
and
stack.
The text segment is read-only and contains the machine
instructions of a program.
The data and stack segments are both readable and writable.
The data segment contains the
initialized and uninitialized data portions of a program, whereas
the stack segment holds the application's run-time stack.
On most machines, the stack segment is extended automatically
by the kernel as the process executes.
A process can expand or contract its data segment by making a system call,
whereas a process can change the size of its text segment
only when the segment's contents are overlaid with data from the
filesystem, or when debugging takes place.
The initial contents of the segments of a child process
are duplicates of the segments of a parent process.

The entire contents of a process address space do not need to be resident
for a process to execute.
If a process references a part of its address space that is not
resident in main memory, the system
pages
the necessary information into memory.
When system resources are scarce, the system uses a two-level
approach to maintain available resources.
If a modest amount of memory is available, the system will take
memory resources away from processes if these resources have not been
used recently.
Should there be a severe resource shortage, the system will resort to
swapping
the entire context of a process to secondary storage.
The
demand paging
and
swapping
done by the system are effectively transparent to processes.
A process may, however, advise the system
about expected future memory utilization as a performance aid.

2.5.1.Â BSD Memory-Management Design Decisions

The support of large sparse address spaces, mapped files,
and shared memory was a requirement for 4.2BSD.
An interface was specified, called
mmap,
that allowed unrelated processes to request a shared
mapping of a file into their address spaces.
If multiple processes mapped the same file into their address spaces,
changes to the file's portion of an address space
by one process would be reflected
in the area mapped by the other processes, as well as in the file itself.
Ultimately, 4.2BSD was shipped without the
mmap
interface, because of pressure to make other features, such as
networking, available.

Further development of the
mmap
interface continued during the work on 4.3BSD.
Over 40 companies and research groups participated
in the discussions leading to the revised architecture
that was described in the Berkeley Software Architecture Manual
[McKusick et al, 1994].
Several of the companies have implemented the revised interface
[Gingell et al, 1987].

Once again, time pressure prevented 4.3BSD from providing an
implementation of the interface.
Although the latter could have been built into the existing
4.3BSD virtual-memory system,
the developers decided not to put it in because
that implementation was nearly 10 years old.
Furthermore, the original virtual-memory design was based
on the assumption that computer
memories were small and expensive, whereas disks were
locally connected, fast, large, and inexpensive.
Thus, the virtual-memory system was designed to be frugal
with its use of memory at the expense of generating extra disk traffic.
In addition, the
4.3BSD implementation was riddled with
VAX
memory-management hardware dependencies that impeded its portability
to other computer architectures.
Finally, the virtual-memory system was not designed
to support the tightly coupled
multiprocessors that are becoming
increasingly common and important today.

Attempts to improve the old implementation incrementally
seemed doomed to failure.
A completely new design,
on the other hand,
could take advantage of large memories,
conserve disk transfers,
and have the potential to run on multiprocessors.
Consequently, the virtual-memory system was completely replaced in 4.4BSD.
The 4.4BSD virtual-memory system
is based on the Mach 2.0 VM system
[Tevanian, 1987].
with updates from Mach 2.5 and Mach 3.0.
It features
efficient support for sharing,
a clean separation of machine-independent and machine-dependent features,
as well as (currently unused) multiprocessor support.
Processes can map files anywhere in their address space.
They can share parts of their address space by
doing a shared mapping of the same file.
Changes made by one process are visible in the address space of
the other process, and also are written back to the file itself.
Processes can also request private mappings of a file, which prevents
any changes that they make from being visible to other processes
mapping the file or being written back to the file itself.

Another issue with the virtual-memory system is the way that
information is passed into the kernel when a system call is made.
4.4BSD always copies data from the process address space
into a buffer in the kernel.
For read or write operations
that are transferring large quantities of data,
doing the copy can be time consuming.
An alternative to doing the copying is to remap the
process memory into the kernel.
The 4.4BSD kernel always copies the data for several reasons:

Often, the user data are not page aligned and are not a multiple of
the hardware page length.

If the page is taken away from the process,
it will no longer be able to reference that page.
Some programs depend on the data remaining in the
buffer even after those data have been written.

If the process is allowed to keep a copy of the page
(as it is in current 4.4BSD semantics),
the page must be made
copy-on-write.
A copy-on-write page is one that is protected against being written
by being made read-only.
If the process attempts to modify the page,
the kernel gets a write fault.
The kernel then makes a copy of the page that the process can modify.
Unfortunately, the typical process will immediately
try to write new data to its output buffer,
forcing the data to be copied anyway.

When pages are remapped to new virtual-memory addresses,
most memory-management hardware requires that the hardware
address-translation cache be purged selectively.
The cache purges are often slow.
The net effect is that remapping is slower than
copying for blocks of data less than 4 to 8 Kbyte.

The biggest incentives for memory mapping are the needs for
accessing big files and for passing large quantities of data
between processes.
The
mmap
interface provides a way for both of these tasks
to be done without copying.

2.5.2.Â Memory Management Inside the Kernel

The kernel often does allocations of memory that are
needed for only the duration of a single system call.
In a user process, such short-term
memory would be allocated on the run-time stack.
Because the kernel has a limited run-time stack,
it is not feasible to allocate even moderate-sized blocks of memory on it.
Consequently, such memory must be allocated
through a more dynamic mechanism.
For example,
when the system must translate a pathname,
it must allocate a 1-Kbyte buffer to hold the name.
Other blocks of memory must be more persistent than a single system call,
and thus could not be allocated on the stack even if there was space.
An example is protocol-control blocks that remain throughout
the duration of a network connection.

Demands for dynamic memory allocation in the kernel have increased
as more services have been added.
A generalized memory allocator reduces the complexity
of writing code inside the kernel.
Thus, the 4.4BSD kernel has a single memory allocator that can be
used by any part of the system.
It has an interface similar to the C library routines
malloc
and
free
that provide memory allocation to application programs
[McKusick & Karels, 1988].
Like the C library interface,
the allocation routine takes a parameter specifying the
size of memory that is needed.
The range of sizes for memory requests is not constrained;
however, physical memory is allocated and is not paged.
The free routine takes a pointer to the storage being freed,
but does not require the size
of the piece of memory being freed.